1. Field of the Invention
The present invention relates to digital printing apparatus and methods, and more particularly to lithographic printing plate constructions that may be imaged on- or off-press using digitally controlled laser output.
2. Description of the Related Art
Traditional techniques of introducing a printed image onto a recording material include letterpress, flexographic and gravure printing, and offset lithography. All of these printing methods require a printing member, usually loaded onto or integral with a plate cylinder of a rotary press for efficiency, to transfer ink in the pattern of the image. In letterpress and flexographic printing, the image pattern is represented on the printing member in the form of raised areas that accept ink and transfer it onto the recording medium by impression; flexographic systems, which utilize elastomeric surfaces, have received more widespread acceptance due to the broad variety of compatible substrates and the ability to run with fluid inks. Gravure printing cylinders, in contrast to raised-surface systems, contain series of wells or indentations that accept ink for deposit onto the recording medium; excess ink must be removed from the cylinder by a doctor blade or similar device prior to contact between the cylinder and the recording medium.
In the case of offset lithography, the image is present on a plate or mat as a pattern of ink-accepting (oleophilic) and ink-repellent (oleophobic) surface areas. In a dry printing system, the plate is simply inked and the image transferred onto a recording material; the plate first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening (or "fountain") solution to the plate prior to or in conjunction with inking. The ink-repellent fountain solution prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas.
If a press is to print in more than one color, a separate printing plate corresponding to each color is required. Such plates have traditionally been imaged off-press, using a photographic process. In addition to preparing the appropriate plates for the different colors, the operator must mount the plates properly on the plate cylinders of the press, and coordinate the positions of the cylinders so that the color components printed by the different cylinders will be in register on the printed copies. Each set of cylinders associated with a particular color on a press is usually referred to as a printing station.
Photographic platemaking processes tend to be time-consuming and require facilities and equipment adequate to support the necessary chemistry. To circumvent these shortcomings, practitioners have developed a number of electronic alternatives to plate imaging. With these systems, digitally controlled devices alter the ink-receptivity of blank plates in a pattern representative of the image to be printed. Such imaging devices include sources of electromagnetic-radiation pulses, produced by one or more laser or non-laser sources, that create chemical changes on plate blanks (thereby eliminating the need for a photographic negative); ink-jet equipment that directly deposits ink-repellent or ink-accepting spots on plate blanks; and spark-discharge equipment, in which an electrode in contact with or spaced close to a plate blank produces electrical sparks to physically alter the topology of the plate blank, thereby producing "dots" which collectively form a desired image (see, e.g., U.S. Pat. No. 4,911,075).
U.S. Pat. Nos. 5,339,737 and 5,379,698, the entire disclosures of which are hereby incorporated by reference, disclose a variety of lithographic plate configurations for use with imaging apparatus that operate by laser discharge (see, e.g., U.S. Pat. No. 5,385,092 and allowed U.S. application Ser. No. 08/376,766). These include "wet" plates that utilize fountain solution during printing, and "dry" plates to which ink is applied directly. These plates may be imaged on a stand-alone platemaker or directly on-press.
In the former case, although the most cumbersome aspects of traditional platemaking are avoided, plates must be manually (and sequentially) loaded onto the platemaker, imaged, inspected, then transferred to the press and mounted to their respective plate cylinders. This involves a substantial amount of handling that can damage the plate, which is vulnerable--both before and after it is imaged--to damage from abrasion. Indeed, even fingerprints can interfere with plate performance by altering the affinity characteristics of the affected areas.
The ability to image on-press obviously reduces the possibility of handling damage substantially, but does not eliminate it. Plates must still be removed from their packaging and mounted to the press; in the case of ablation-type plates, it is frequently necessary to clean the plates to remove imaging debris, an operation that can result in abrasion if performed improperly. Indeed, lithographic printing plates can suffer damage even without handling: airborne debris, environmental contamination, movement of the packaged plates and the mere passage of time can inflict various stresses that interfere with ultimate plate performance.
To protect the plate during packaging, shipment and use, manufacturers may add a peelable barrier sheet to the final construction. As discussed, for example, in the '737 patent, this layer adheres to the surface of the plate, protecting it against damage and environmental exposure, and may be removed following imaging. Unfortunately, this sheet can itself damage the plate if the degree of adhesion is inappropriate or if carelessly removed, and in any case adds cost to the plate and its removal imposes an additional processing step.